Eco-friendly flame retardant poly(lactic acid) composites based on banana peel powders and phytic acid: flame retardancy and thermal property

Exploiting Waste towards More Sustainable Flame-Retardant Solutions for Polymers: A Review

The development of sustainable flame retardants is gaining momentum due to their enhanced safety attributes and environmental compatibility. One effective strategy is to use waste materials as a primary source of chemical components, which can help mitigate environmental issues associated with traditional flame retardants. This paper reviews recent research in flame retardancy for waste flame retardants, categorizing them based on waste types like industrial, food, and plant waste. The paper focuses on recent advancements in this area, focusing on their impact on the thermal stability, flame retardancy, smoke suppression, and mechanical properties of polymeric materials. The study also provides a summary of functionalization methodologies used and key factors involved in modifying polymer systems. Finally, their major challenges and prospects for the future are identified.

A nanocellulose-based flexible multilayer sensor with high sensitivity to humidity and strain response for detecting human motion and respiration

Biomass-based flexible sensors with excellent mechanical and sensing properties have attracted significant attention. In this study, based on the excellent dispersibility and degradability of nanocellulose crystals, we designed a polyvinyl alcohol/nanocellulose crystals/phytic acid (PCP) composite film with good flexibility and high sensitivity to humidity. A layer of multiwalled carbon nanotubes (MWCNT) and nanocellulose crystals (CNC) was further sandwiched between two PCP layers as a flexible multifunctional sensor (PCPW) to detect human movement and respiration. Phytic acid contains abundant phosphate groups that enhance proton conduction, allowing the PCPW composite film to change its electrical resistance in a sensitive and repeatable manner when the relative humidity was varied between 35 %-93 %. Meanwhile, CNC derived from sisal fibers enhanced the PCPW sensor's conductivity (3.3 S/m) and mechanical properties (elongation at break: 99 %) by improving the dispersion and connectivity of MWCNT. The PCPW sensor displayed a high sensitivity to strain (gauge factor: 49.5) and could monitor both facial expressions (smiling and winking) and the bending of joints. The sensor also generated stable electrical responses during breathing and blowing due to the change in humidity. Therefore, this biodegradable and multifunctional sensor has good application prospects.

Utilization of spent coffee grounds as charring agent to prepare flame retardant poly(lactic acid) composites with improved toughness

The objective was to utilize spent coffee grounds (SCG) as charring agent to combine with ammonium polyphosphate (APP) to prepare flame retardant poly(lactic acid) (PLA) composites with improved toughness. PLA/APP-SCG and PLA/APP-SCG/KH560 composites were prepared, and silane coupling agent KH560 was applied to improve particle-matrix interfacial compatibility. The particle-matrix interface, char formation, flame retardancy, mechanical properties and fracture morphology of PLA composites were studied. Results showed that PLA/APP-SCG5% and PLA/APP-SCG20% passed UL-94 V-0 rating, and increase in charred residues was favorable for improving flame retardancy. Improved toughness was also obtained compared to PLA, attributed to debonding of APP from matrix under external force as well as plasticization effect of coffee oil contained in SCG. PLA/APP-SCG5%/KH560 and PLA/APP-SCG20%/KH560 showed smaller elongation at break and impact strength compared to PLA/APP-SCG5% and PLA/APP-SCG20%, respectively. The improved interfacial compatibility was unfavorable for debonding of APP from matrix, and both APP and SCG played the role of enhancing strength, thus decreasing toughness. PLA/APP-SCG/KH560 counterparts were actually set as parallel samples to prove that PLA/APP-SCG composites showed improved toughness with weak interfacial compatibility. This study has provided a practical approach to utilize bio-derived wastes as charring agent to prepare flame retardant PLA composites with enhanced toughness.

Biodegradable ion-conductive polyvinyl alcohol/okra polysaccharide composite films for fast-response respiratory monitoring sensors

Polyvinyl alcohol (PVA) and okra polysaccharide (OP) are biodegradable polymers with high hydrophilicity and good biocompatibility with potential for use as flexible humidity-sensitive materials. Herein, biodegradable flexible composite films (named POP films) were prepared from PVA, OP, and phytic acid using a solution-casting method based on. POP films exhibited excellent mechanical strength, flexibility, flame retardancy, water resistance, humidity response, and humidity-sensing characteristics. Notably, the POP humidity sensors exhibited a hysteresis value of 1.88 % relative humidity for the adsorption and desorption processes and good sensitivity over a wide humidity range of 35-95 %. In addition, the humidity sensor distinguished the frequency of nose breathing, and its response and recovery times were 0.9 and 1.98 s, respectively. The excellent performance of POP sensors in monitoring humidity and human respiratory rates demonstrates the sensor's potential for wearable smart devices.

Self-assembled biobased chitosan hybrid carrying N/P/B elements for polylactide with enhanced fire safety and mechanical properties

The inherent shortcomings such as flammability, brittleness, and low crystallinity limit the broad applications of poly(lactic acid) (PLA). To improve the fire resistance and mechanical properties of PLA, a chitosan-based core-shell flame retardant additive (APBA@PA@CS) was prepared for PLA via the self-assembly of interionic interactions among chitosan (CS), phytic acid (PA), and 3-aminophenyl boronic acid (APBA). The peak heat release rate (pHRR) and total heat release rate (THR) of PLA composite containing 3 wt% APBA@PA@CS decreased from 460.1 kW/m² and 75.8 MJ/m² to 419.0 kW/m² and 53.1 MJ/m², respectively. The presence of APBA@PA@CS contributed to the formation of a high-quality char layer rich in phosphorus and boron in the condensed phase and released non-flammable gases in the gas phase to hinder the exchange of heat and O₂, thereby having a synergistic flame retardant effect. Meanwhile, the tensile strength, elongation at break, impact strength, and crystallinity of PLA/APBA@PA@CS were increased by 3.7 %, 17.4 %, 5.3 %, and 55.2 %, respectively. This study provides a feasible route to construct a chitosan-based N/B/P tri-element hybrid to improve the fire safety performance and mechanical properties of PLA biocomposites.

Fabrication of Phytic Acid/Urea Co-Modified Bamboo Biochar and Its Application as Green Flame Retardant for Polylactic Acid Resins

It is of great significance to develop green, sustainable additives to improve the thermal stability and flame retardancy of biopolymers. In this work, a synergistic modification of P/N elements to bamboo biochar (mBC) was successfully achieved by grafting a reaction of phytic acid and urea with preoxidized bamboo biochar. Fourier transform infrared spectroscopy, X-ray diffraction, nuclear magnetic resonance and scanning electron microscope determinations of the mBC demonstrated a successive grafting of phytic acid and urea to the originally porous surface. The ground mBC was blended with polylactic acid (PLA) to prepare mBC/PLA composites by extrusion and hot pressing. Mechanical strength studies showed a compromise in rigidity, which might originate from the mBC overdose and its limited miscibility with the resin. The thermogravimetric results supported the fact that the enhancement of thermal stability and flame retardancy of the composites with the mBC dosage, which showed that the mBC dosage in the PLA composites was not only lower than that of the conventional flame retardants, but also outperformed the counterparts using BC modified by inorganic phosphoric acid and urea. The mBC was prone to accelerate the earlier decomposition of the composites (30 °C lower in decomposition) and generate a continuous, dense residual carbon layer, which provides an effective shield resisting the mass and heat transfer between the combustion area and the underlying composite matrix. Only 10 wt% of mBC dosage could achieve a V-0 rating (UL94) for the composite, with a higher limiting oxygen index up to 28.3% compared to 20.7% for that of the virgin PLA; the cone colorimetric results also suggested that the flame retardancy had been greatly improved for all composites. In this work, biobased P-/N-containing bamboo biochar would be expected as a nontoxic biochar-based flame retardant that serves as green filler in polymer composites.

Flame Retardancy and Thermal Property of Environment-Friendly Poly(lactic acid) Composites Based on Banana Peel Powder

Banana peel powder (BPP) was used to prepare poly(lactic acid) (PLA) bio-based composites and the flame retardancy was enhanced by introducing silica-gel microencapsulated ammonium polyphosphate (MCAPP). The results showed that the limiting oxygen index (LOI) of PLA containing 15 wt % BPP was 22.1% and just passed the UL-94 V-2 rate. Moreover, with the introduction of 5 wt % MCAPP and 15 wt % BPP, the PLA composite had a higher LOI value of 31.5%, and reached the UL-94 V-0 rating, with self-extinguishing and anti-dripping abilities. The PLA/M5B15 also had a lower peak heat release rate (296.7 kW·m^-2), which was 16% lower than that of the PLA/B15 composite. Furthermore, the synergistic effects between MCAPP and BPP impart better thermal stability to PLA composites. According to the investigation of the char residue and pyrolysis gaseous products, MCAPP with BPP addition is beneficial to the formation of a higher quality char layer in the solid phase but also plays the flame retardant effect in the gas phase. This work provides a simple and efficient method to solve the high cost and flammability issues of PLA composites.

Facile Preparation of a Novel Vanillin-Containing DOPO Derivate as a Flame Retardant for Epoxy Resins

A novel bio-based flame retardant designated AVD has been synthesized in a one-pot process via the reaction of 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO), vanillin (VN), and 2- aminobenzothiazole (ABT). The structure of AVD was confirmed using Fourier transform infrared spectroscopy (FTIR), and ¹H and ³¹P nuclear magnetic resonance spectroscopy (NMR). The curing process, thermal stability, flame retardancy, and mechanical properties of the epoxy resin (EP) modified with AVD have been investigated comprehensively. The extent of curing, the glass transition temperature and the crosslinking density of the blend decreased gradually with increasing AVD content. The thermogravimetric analysis (TGA) was used to demonstrate that the presence of AVD reduced the thermal decomposition rate for EP and enhanced the formation of carbon residue during resin decomposition. A blend of 7.5 wt% AVD (0.52% phosphorus) displays a UL-94V-0 rating and a LOI of 31.1%. Reduction of the peak heat release rate, total heat release rate and total smoke production was 41.26%, 35.70%, and 24.03%, respectively, as compared to the values for pure EP. The improved flame retardancy of the flame retardant epoxy (FREP) may be attributed to the formation of a compact and continuous protective char layer into the condensed phase as well as the release of non-combustible gases and phosphorus-containing radicals from the decomposition of AVD in the gas phase. AVD is a new and efficient biobased flame retardant for epoxy with great prospects for industrial applications.

Governing effects of melt viscosity on fire performances of polylactide and its fire-retardant systems

Extreme flammability of polylactide (PLA) has restricted its real-world applications. Traditional research only focuses on developing new effective fire retardants for PLA without considering the effect of melt viscosity on its fire performances. To fill the knowledge gap, a series of PLA matrices of varied melt flow index (MFI) with and without fire retardants are chosen to examine how melt viscosity affects its fire performances. Our results show that the MFI has a governing impact on fire performances of pure PLA and its fire-retardant systems if the samples are placed vertically during fire testing. PLA with higher MFI values achieves higher limiting oxygen index (LOI) values, and a lower loading level of fire retardants is required for PLA to pass a UL-94 V-0 rating. This work unveils the correlation between melt viscosity and their fire performance and offers a practical guidance for creating flame retardant PLA to extend its applications.

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The polymer melt viscosity plays a governing role in fire performances of PLA

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PLA with higher MFI shows higher LOI and needs less FR to get the UL-94 V-0

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A critical MFI range of 16.5–29.9 g/10min is found to affect the fire performance

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Melt viscosity does not affect fire performances when PLA was tested horizontally